This invention relates to a driving device (a linear actuator) and, in particular, to a driving method for driving a driving device where an electro-mechanical transducer such a piezoelectric element is used as a driving source of the driving device (the linear actuator).
Previously, linear actuators (driving devices) using electro-mechanical transducers such as piezoelectric elements, electrostrictive elements, magnetostrictive elements, or the like are used as auto-focus actuators or zoom actuators for use in cameras.
By way of illustration, Japanese Patent No. 2633066 (JP-B 2633066) (which will be also called a first patent document), which corresponds to U.S. Pat. No. 5,225,941, discloses a driving device comprising a driving rod frictionally engaged with a lens barrel, a piezoelectric element disposed in contact with the driving rod, and a leaf spring for bringing the driving rod into frictional engagement with the lens barrel. That is, the driving rod is bonded to an end of the piezoelectric element in an expansion direction. The lens barrel is movably supported to the driving rod. The leaf spring produces friction between the driving rod and the lens barrel. In the driving device disclosed in JP-B 2633066, a voltage is applied to the piezoelectric element so as to make a speed of expansion of the piezoelectric element different from a speed of contraction thereof.
In addition, Japanese Patent No. 3218851 (JP-B 3218851) (which will be also called a second patent document), which corresponds to U.S. Pat. No. 5,589,723, discloses a driving apparatus comprising a piezoelectric element, a driving member (a driving shaft), coupled to the piezoelectric element, for extending in an expansion direction of the piezoelectric element, and a driven member (a moving member, a lens barrel) having a friction member frictionally coupled to the driving member (the driving shaft). The driving apparatus in JP-B 3218851 drives the lens barrel by devising a driving signal applied to the piezoelectric element.
Japanese Unexamined Patent Application Publication No. H9-191665 (JP-A 9-191665) (which will be also called a third patent document), which corresponds to U.S. Pat. No. 5,890,391, discloses a linear drive mechanism using an electro-mechanical transducer which is insensitive to elastic deformation of a constituent member. The linear driving mechanism disclosed in JP-A 9-191665 comprises the electro-mechanical transducer, a working member which is fixedly connected to the electro-mechanical transducer and which displaces with the electro-mechanical transducer, a driving member frictionally coupled to the working member, a driven member coupled to the driving member, and a driving pulse generating arrangement for causing expansion/contraction displacement to the electro-mechanical transducer. In JP-A 9-191665, an internal barrel is integratedly and fixedly attached to a fixed lens barrel. The internal barrel has extensions which are formed in a radial direction on part of the internal barrel. A drive shaft (a moving portion) arranged parallel to an optical axis is supported by bearings of the extensions so as be movable in the optical axis direction. A piezoelectric element (the electro-mechanical transducer) is disposed between the working member (a vibration friction portion) and the extension of the internal barrel. The top half of the working member touches the drive shaft and a pad is installed on the bottom half of the working member and exerts a force toward the drive shaft via a spring, such that the pad touches the bottom half of the working member. The working member and the pad are friction-bonded to the drive shaft via a suitable friction force. By generating expansion/contraction displacements having different rates in the thickness direction of the piezoelectric element, the working member is displaced, and the drive shaft is driven by friction contact with the working member together with a lens holder frame serving as the driven member.
Japanese Unexamined Patent Application Publication No. 2006-304529 (JP-A 2006-304529) (which will be also called a fourth patent document) discloses a high-performance drive device whose cost and weight can be reduced in comparison with a structure with a metal material, and for which a moving member of high rigidity is used without deteriorating the speed of movement and a drive force. In the drive device disclosed in JP-A 2006-304529, the moving member is formed with a liquid crystal polymer including carbon fibers. The drive device disclosed in JP-A 2006-304529 comprises a piezoelectric element for expanding and contracting in response to an applied voltage, a drive shaft (a vibration friction portion) fixed to one end of the piezoelectric element in an expansion/contraction direction, the moving member (a moving portion) which is slidably and frictionally engaged with the drive shaft, and a weight (a stationary member) connected to another end of the piezoelectric element in the expansion/contraction direction. By vibrating the drive shaft by changing a speed or an acceleration of the piezoelectric element in the expansion/contraction direction, the drive device moves the moving member (the moving portion) along the drive shaft (the vibration friction portion).
Furthermore, Japanese Patent No. 3002890 (JP-B 3002890) (which will be also called a fifth patent document) discloses an ultrasonic linear motor which is simple in structure and which comprises a simple driving circuit. The ultrasonic linear motor disclosed in JP-B 3002890 comprises a fixed base (a stationary member), a piezoelectric element (an electro-mechanical transducer), a frictional member (a vibration friction portion), and a moving element (a moving portion). The electro-mechanical transducer has one end secured to the fixed base and another end mounted with the frictional member extending laterally over the piezoelectric element. The piezoelectric element carries out, in an expansion/contraction direction, an expansion/contraction motion which makes the frictional member an expansion/contraction motion. The moving element is movable in parallel with the expansion/contraction direction of the piezoelectric element. The moving element is brought into pressure contact with the frictional member. The piezoelectric element is applied with a voltage having a saw-tooth waveform so that moving speeds of to-and-fro movements are different from each other in the expansion/contraction motion.
FIG. 10 is a waveform chart for use in describing a related art driving method of the driving devices disclosed in the first through the fifth patent documents. In FIG. 10, the abscissa represents a time instant and the ordinate represents a displacement. In FIG. 10, Wd(v) represents a displacement waveform of the vibration friction portion and Wd(m) represents a displacement waveform of the moving portion. Generally, the related art driving method comprises changing speeds of vibrations of the electro-mechanical transducer on expanding and on contracting, making the vibration friction portion a saw-tooth displacement, and moving the moving portion.
In order to obtain the saw-tooth displacement to the vibration friction portion, the electro-mechanical transducer is applied with the voltage having the saw-tooth waveform. Alternatively, by applying a voltage having a rectangular wave to the electro-mechanical transducer, it is necessary to obtain the saw-tooth displacement to the vibration friction portion due to a characteristic of a transfer function which is determined by structure of the driving device. Therefore, the structure of the driving device is closely related to a frequency where the saw-tooth displacement is obtained to the vibration friction portion.
However, in a method of producing a difference between displacement speeds of the vibration friction portion on expanding and on contracting the electro-mechanical transducer such as a related art driving method of the driving device, it may be difficult to obtain displacements having different speeds to the vibration friction portion caused by the structure of the driving device.